Very cheap light source for split grade contact printing

[paolod]

I picked up this USB powered RGB LED strip at Walmart for $10. It's 6' long with 60 LEDs. I cut it up and soldered it back together in a flat grid but for contact printing it's probably fine to just leave it as is and put it far enough away that the light hitting the paper is even.


It comes with an IR remote control with buttons to set the color and brightness. It remembers the last setting when it's turned off. So it can be turned on and off, switched between blue and green, and it will maintain the same brightness setting. It's easy to use as long as you don't hit the wrong button and make it start flashing white!

A quick test on Ilford MGIV RC paper shows the green and blue seem to work to control contrast. I compared to some old Ilford grade 00 and grade 5 filters with an incandescent bulb in an enlarger.


It also supports Bluetooth for control by a mobile app, and the protocol has been reverse engineered. So one could make a phone or PC app to control timing and contrast, maybe to make test strips automatically. Probably could even get it to work with voice control to avoid typing in numbers.

 

[ic-racer]

Nice. I presume these are inexpensive, so another thing to do might be to cut up two of them and intermingle them. So set each controller to either blue or green and connect each to a timer. Then one could control contrast and density with the two timers.

 

[paolod]

It shouldn't be necessary to get two strips. The strip has a separate pin for each color so timers could control the green and blue independently. The brightness of each color can also be set separately with a Bluetooth command. Probably could also leave the red on all the time as a safelight, although I haven't checked whether or not it fogs paper.

 

[koraks]

although I haven't checked whether or not it fogs paper.

Give it a try, but I expect it will fog, and quite badly too.

 

[radiant]

Give it a try, but I expect it will fog, and quite badly too.

I assume these are basically in same wavelength as WS2812 leds and WS2812 should be at 620-630. Ilford MG RC V sensitivity drops to zero at 550 so I guess there is enough margin too.

Which reminds me of testing my WS2812 safelights for fogging (after two years of use) .. :-/

 

[radiant]

But I think this is a very very cheap conversion "kit" to convert to LED enlarger. Maybe some old / cheap tablet to control the lights ("permanent" / ergonomic install to darkroom) and you get really advanced stuff going on!

Just a sidenote: I have converted my enlarger to LED with LED strips with really decent results, more about it here: https://www.photrio.com/forum/threads/enlarger-conversion-to-led-ws2812-fujimoto-g70.179537/

 

[koraks]

I assume these are basically in same wavelength as WS2812 leds and WS2812 should be at 620-630. Ilford MG RC V sensitivity drops to zero at 550 so I guess there is enough margin too.

That's the theory. In practice, I found that red leds always emit a secondary band in the green area of the spectrum, with lesser emissions ranging all the way from blue (minor) to orange (more so). The same is true for blue and green leds; they also have secondary emission/limited spectral purity. For some reason, the problem seems less of an issue with those, at least for B&W printing. Datasheets, when available, do not always accurately show these secondary emissions. I also found that 660 'deep red' leds seem to work a little better in terms of spectral purity. However, in RGB led products, the red is *always* ~620 and *never* 660 ('deep red'), at least in the hundreds of products I have swiped through to date. Also, the blue is *always* ~460 and *never* 440-450 ('royal blue') where you'd like it to be for darkroom printing (both B&W and color).

All the ready-made RGB products will work to an extent, but are not optimal in my opinion.

 

[radiant]

That's the theory. In practice, I found that red leds always emit a secondary band in the green area of the spectrum, with lesser emissions ranging all the way from blue (minor) to orange (more so).

Ah, good to know. I understand then why always-on red is bad idea. My reds turn off when exposing, but that's quite small decrease in percentage. Anyways my papers are under red light about 2-3 minutes maximum so probably the fogging is really minor. Or I hope it is. I need to test!

 

[distributed]

A cool idea. The test strips indicate that everything is working as intended. It's kinda hard to tell, but I'd venture to say that your green LEDs offer a softer grade (larger range) than the Ilford 00 filter. This matches my experience with green LEDs on MGRC paper.

About the safelighting: Sure, give it a try. But I second vedostuu by guessing that you'll be fine with the red LEDs. Cheap, non-special red LEDs should be quite above 600 nm and thus well away from even the Ilford safelight cutoff at 580 nm. I expect you to be safe if you are not using them at insane brightness.

 

[distributed]

Koraks, just read your post now. Interesting observation! Do you maybe have a graph of a "typical" LED showing this secondary band?

 

[mnemosyne]

The sensitivity of some variable contrast paper like Foma Variant extends much further into the red end of the spectrum than Ilford MG. Special care has to be taken. Secondary spectra of normal red LEDs, although rather weak, can easily fog this paper. Test thoroughly for fogging (sensitizing the paper by pre-exposure is a must!) to determine correct distance of light source and safe handling times.

I guess this effects can not be solely determined by merely looking at charts or graphs of papers and light sources, as these representations are often idealized or simplified, or the graph is simply cut off at certain thresholds.

 

[koraks]

Koraks, just read your post now. Interesting observation! Do you maybe have a graph of a "typical" LED showing this secondary band?

Unfortunately, no, but you can quite easily check for yourself by holding a cd/dvd at an angle towards a red led (in an otherwise dark room) and see the spectrum it emits. You'll get a rainbow of colors in which red is dominant, but yellow and green are visibly present. I found this to be a problem in two ways: (1) when using red leds (strips) as darkroom safelight, I witnessed severe fogging issues unless the leds are filtered with some rubylith material which cuts down the fog by a few stops. And also (2) when constructing a led enlarger for color work, I noticed that 620nm red leds gave problems with impure hues in parts of the spectrum where you wouldn't expect it; in other words, they didn't only activate the cyan dye, but also the other dyes. The problem is much less and actually manageable/acceptable with 660nm leds.

For B&W work, fortunately the problem is fairly easy to deal with, by limiting the exposure of paper to red led light and by performing a simple fogging test under the red led light that will be used.

 

[MattKing]

My 16 foot red LED rope light safelight works great with Ilford and Oriental variable contrast papers, passing the full Kodak safelight test with no fogging.
I have it strung along the juncture between the wall and the ceiling on three sides (mostly) of my bathroom darkroom.

 

[koraks]

Lucky you! I had to go all creative with rubylith and tape on mine. If you get your strips from China, it isnt a very expensive experiment to get a product that works well, I suppose. I just stuck with the first 5 meters of strip I received. It's a lot of light btw, which of course is a blessing and a curse at the same time.

 

[MattKing]

Mine aren't strips per se
They are like these ones: https://www.homedepot.com/p/DEERPOR...ion-2-Pack-32-ft-Total-RL16-RD-2PK/206792268?

 

[eli griggs]

Does anyone here use either a colour meter, like a Minolta, or a colour analyzer like my PM 2, to check the light output to the easel, each time or even just the once on instillation?

 

[koraks]

Mine aren't strips per se
They are like these ones: https://www.homedepot.com/p/DEERPOR...ion-2-Pack-32-ft-Total-RL16-RD-2PK/206792268?

The link doesn't work for me, probably because I'm not in the US (?)
I found the same ones on Amazon though; I see what you mean. They look nice. Useful for others from the US to know that these work well!

Does anyone here use either a colour meter, like a Minolta, or a colour analyzer like my PM 2, to check the light output to the easel, each time or even just the once on instillation?

For what purpose, exactly? To measure color, or light intensity, or both?

 

[MattKing]

The link doesn't work for me, probably because I'm not in the US (?)
I found the same ones on Amazon though; I see what you mean. They look nice. Useful for others from the US to know that these work well!

I'm actually in Canada. I can't remember if I ordered mine from the US, because pre-Covid that was something I could do easily, as I have a US shipping address.
This is nearly the best time of year to order - availability of these red LEDs is best around the Christmas season!

 

[radiant]

I just tested RED channel of WS2812 with Ilford MG RC V for 23 minutes from 20 centimeter distance. Absolutely no sign of fogging.

I'm not saying that that other papers are fog safe with that LED type but at least one paper passed fogging test completely. I need to test Foma papers next time I print on those.

 

[koraks]

I haven't tried Ilford;most of the testing I did was with Adox. And perhaps those leds are just better; I suspect they are given your test. That wouldbe good news!

 

[MattKing]

WRT your testing, did you test with both pre-exposure and post exposure, as per the Kodak test?
I know, it is about the most boring test around, but it really does reveal more than a simple straight test.
In case you don't have the link: https://www.kodak.com/uploadedfiles..._motion_products_filter_K4_Safelight_1106.pdf